Allow account code to be updated

It should be possible to update an account's code if it's type is `AccountType::RegularAccountUpdatableCode`.

We have a rudimentary setup for this in the tx kernel (e.g. `NEW_CODE_ROOT_PTR`) but no APIs for users to set it.

Updating the code to the new commitment is currently done in the epilogue so that the _next_ transaction would start using the new code.

This change would also require including the commitment to the new code in the account delta.